Tobacco smoking is a major health problem leading each year to $50 billion dollars in health costs and 400,000 deaths in the United States. Significant evidence indicates that nicotine is the component in tobacco leading to abuse and addiction. Nicotine binds to nicotinic receptors found in the brain. To characterize the molecular biology of nicotine addiction, our research will focus on nicotinic receptors in the brain and how nicotine-induced changes in nicotinic receptors correlates with aspects of addiction. Like other drugs of abuse, many of the addictive effects of nicotine appear to result from the stimulation of dopaminergic, mesolimbic neurons, in particular dopaminergic neurons from the ventral tegmental area (VTA). The short-term effects of nicotine are caused by the activation of nicotinic receptors on these neurons. Long-term exposure to nicotine enhances or "sensitizes" the response of these neurons to nicotine, which correlates with the enhancement or "upregulation" of high-affinity binding to nicotinic receptors. The goals of this proposal are: 1) to identify nicotinic receptor subtypes involved in nicotine upregulation, 2) to identify mechanisms involved in nicotine upregulation of different nicotinic receptor subtypes and 3) to test whether nicotinic receptor upregulation by nicotine is involved in nicotine-induced long-term potentiation and behavorial/chemical sensitization as a collaboration with Projects 2 and 3. The hypotheses to be tested are that: 1) nicotinic receptor upregulation in heterologous expression systems and in vivo is caused by a receptor state change that causes the receptors to enter a hypersensitive or functionally upregulated state;2) nicotine-induced long-term potentiation and sensitization of the nicotine response are initiated, at least in part, by receptor upregulation. Several possible nicotinic subunit combinations will be expressed to test whether upregulation occurs for each combination and the extent to which upregulation is a change in receptor number or entry into a hypersensitive state. Additional experiments will assay for nicotinic receptor upregulation in brain after nicotine exposure in rats. These experiments will test whether nicotine exposure, which induces locomotor sensitization, causes nicotinic receptor upregulation in the brain regions identified with locomotor sensitization and examine which nicotinic receptor subtypes are upregulated in these brain regions.